Conventional radio frequency (RF) heating devices, such as domestic or industrial microwave ovens, use a magnetron to generate RF radiation. Typically, these devices produce radiation at a single frequency or pseudo-randomly within a specific frequency band. These devices are also bulky and are limited with respect to the degree of controllability they provide over the radiation that is introduced into the cavity in which an object is to be heated (e.g. food).
More recently, devices which generate RF radiation using solid state semiconductor components have been proposed. Some conventional devices use a power divider architecture that employ multiple paths of a phase coherent signal for amplification and delivery to a cavity (e.g. see EP 2,182,774 A1 and EP 2,205,043 A1).
Although conceptually simple, these implementations have several drawbacks. For example, the power divider architecture forces all antennae in the device to provide power to the cavity at the same frequency, which may or may not be desired.
Also, in a system having multiple antennae, the antenna locations will tend to be dominated by the physics of the heating cavity. Ideal placement of the antennae, determined by the physics of the cavity, may require relatively long RF distribution paths. Longer distribution paths at the RF frequency create greater losses and may also inhibit phase alignment between the antennae. Because of these issues, the power divider architecture may place unwanted restrictions upon the design of the device.
Some conventional devices seek to adjust the generation of radiation introduced into the cavity of a heating device based on a reflected signal. For example, EP 2,205,043 A1 describes a microwave heating apparatus capable of heating an object by reducing the reflected power generated based on the object to be heated having the different configuration, kind, size, and amount, by arranging a first feeding part in the wall surface of the heating chamber, and arranging a second feeding part to radiate the reflected power received by the first feeding part to the heating chamber again, serving as the plurality of microwave supplying means each having a function to radiate the microwave.
In another example, EP 2,434,837 A1 describes a microwave heating device and microwave heating method in which damage of a microwave generating part by reflected power may be prevented by a control part, which receives a reflected power signal and a supply power signal from a power detecting part. The control part executes a frequency sweep operation of a prescribed frequency band with frequency sweep power lower than rated supply power supplied to a power feeding part during a heating operation, to thereby set an oscillation frequency at which the minimum reflected power becomes minimum, and to control an oscillation frequency of an oscillator part and an output of a power amplifier part.
EP 2,549,832 A1 describes a microwave heating apparatus. U.S. Pat. No. 5,081,425 describes a voltage standing wave ratio (VSWR) adaptive power amplifier system. WO 2009/020530 describes a microwave system generator and controller for gas and liquid chromatography. CN103152889 describes a circuit and control method for controlling power of variable-frequency microwave oven. U.S. Pat. No. 4,504,718 describes a microwave heating apparatus with solid state microwave oscillating device. U.S. Pat. No. 3,953,702 describes a solid state microwave oven power source. US 2004/206755 describes a microwave heating using distributed semiconductor sources.